For a long time specialists have been agreed that in order to be really effective, teeth must be brushed from the gum towards the end of the tooth, i.e. downwards for the top jaw and upwards for the bottom jaw.
There presently exist many types of electric toothbrush, with vibrating, reciprocating, or rotary motion. For example, European patent EP 0 488 971 describes an electric toothbrush having two contrarotating brushes disposed to act on the same side of the dental arch. Under such circumstances, it is clear that although one of the brushes is always operating in the proper direction, the other one is necessarily acting in the wrong direction.
None of those devices can guarantee that brushing will always take place in the best direction, unless an automatic system is incorporated therein for reversing the direction of rotation as a function of the position of the device in the mouth, and such a system would be complex, expensive, and generally unreliable.
To mitigate the above drawbacks, tooth brushing devices are nevertheless known which include cylindrical brushes having parallel axes that are rotated in opposite directions, enabling the teeth that are to be cleaned to be inserted between them so that each side of a tooth is brushed in the root-to-end direction: by way of example of a device of this category, in which the present invention belongs, mention may be made of patent application FR 2 489 120 published on Mar. 5, 1982 which describes such a brushing device in which the cylindrical brushes are spaced apart by a constant distance greater than the maximum width of a tooth (about 5 millimeters as mentioned in the application), and are free at their distal ends (i.e. their ends remote from the handle containing the system for applying rotary drive to said brushes via their opposite ends). Devices of the same type also include patent CH 644256 published on Jul. 31, 1984, in which the brushes are provided with shafts of conical section and made of synthetic material whose resilience enables a certain amount of pressure to be obtained against the teeth that are encompassed between them. To avoid any risk of obtaining an effect that is the opposite of the intended result, devices of that type necessarily include means for controlling the direction of brushing, either by reversing the direction of rotation of the drive motor, or else by means of angled couplings imposing a determined position on the brushes relative to the dental arch to be cleaned.
Mention is also made of patent EP 108097 published on Jan. 20, 1988, which teaches a brushing device having a plurality of interchangeable modules, one of which has frustoconical brushes and another has three parallel-axis cylindrical brushes that are short and of large diameter, with the middle brush serving to brush the tops of teeth and with the assembly being received in a highly-enveloping case: nevertheless such a disposition is not very effective, firstly because the rotary motion of the middle brush opposes that of one of the side brushes thereby impeding proper effectiveness of the middle brush, and secondly because the presence of the middle brush determines a certain application height for the device which does not always enable action to be taken over the full height of teeth all the way to the gum.
In addition to the above-mentioned drawbacks, those known devices generally have constant separation between the brushes, which firstly requires several different heads or several different sizes of device to be proposed depending on the user (children, adults, . . . ), and secondly, even for a single user, does not enable uniform brushing effectiveness to be obtained on all teeth, given the differences in tooth thicknesses (molars, premolars, canines, . . . ); and in devices of the type described in patent CH 644256, which are the only devices enabling the brushes to be moved apart because they are mounted on flexible shafts and held at one end only, in addition to the above-mentioned drawback of controlling brushing direction, they do not enable uniform pressure to be applied to the teeth since the spacing between them cannot adapt itself to the width of teeth over their entire length, and such a disposition necessarily gives rise to bending and shear forces on the drive shafts which are damaged thereby in the long run.
In an attempt to resolve those difficulties, attention may be drawn to patent application FR 2 662 598 published on Dec. 6, 1991, which describes a tooth brushing device of the same type as that described in above-mentioned application FR 2 489 120 having two contra-rotating brushes on rigid shafts, that are free at their distal ends and that are held by their opposite ends via which they are driven: the case that receives them includes a guide in which one of the brush ends is movable away from the other over a circular arc guaranteeing continuity of drive around and by a gear wheel, with the help of a cable-and-spring contrivance; the object of that device is to adapt to teeth of any thickness and to maintain constant pressure thereon in substantially uniform manner over the full length of the brushes which, for that purpose, must remain substantially parallel throughout brushing. This also constitutes one of the objects of the present invention, but the solution taught by that prior document is very complex, is not reliable, and requires action to be taken by the user when the device is placed in the mouth, which tends to discourage use thereof and which means that good results are not guaranteed.